1. Field of the Invention
The present invention relates to an evaporation loss control device provided with an automobile fuel tank, such as a canister.
2. Description of the Art
A fairly large amount of fuel evaporates when it is supplied to an automobile fuel tank through a fuel pump nozzle. Part of the fuel in the fuel tank and the carburetor float chamber also evaporates while the automobile is running or at rest.
In order to prevent the leakage of the evaporated fuel into the atmosphere, a canister (evaporation loss control device) filled with a fuel absorbent is attached to, e.g., the fuel tank. The absorbent in the canister serves to trap the evaporated fuel. Similar devices have been employed to trap fuel evaporating or leaking from fuel storage tanks other than those installed on automobiles. In such evaporation loss control devices, charcoal has been used widely as a fuel absorbent.
However, canisters employing the charcoal become less efficient over time to the point where untrapped fuel vapor is released into the atmosphere at undesirable levels. This is because the working capacity of charcoal (i.e., the ability of charcoal to trap gasoline vapor) decreases significantly when the charcoal is in contact with liquid gasoline. Such contact between charcoal and liquid gasoline occurs when the evaporated fuel condenses to reach the charcoal through flowing on the inner surfaces of the piping around the canister or on the charcoal-free space at the top of the canister.
Another reason that canisters became less efficient is that the working capacity decreases when the component of the fuel having a high boiling point is absorbed by charcoal. The component of the fuel with a low boiling point that is absorbed by charcoal, with a number of carbon atoms being less than or equal to 4 or 5, can be easily desorbed when the devices are subjected to purging. On the contrary, the component having the high boiling point of the fuel cannot be easily desorbed.
In order to cope with the above problem, it has been proposed to use polymer absorbent together with charcoal (see Japanese Patent Application (Laid Open) No. 227861/1989, and U.S. Pat. No. 4,684,382) as the absorbent. In such evaporation loss control devices, the absorbent chamber of the known type is divided into two chambers by a cellular partition wall. One of those chambers is filled with charcoal, and the other is filled with polymer absorbent. The working capacity of the charcoal is kept high by using the polymer absorbent which absorbs the component with high boiling point contained in the fuel vapor and the liquid fuel.
In the above prior art (JPA 227861/1989), however, since the polymer chamber is placed close to the charcoal chamber, separated only by the cellular partition, the evaporated fuel absorbed by the polymer absorbent evaporates as time goes by and it is then absorbed by charcoal in the vicinity of the polymer chamber, thus causing the charcoal to decrease its working capacity. In U.S. Pat. No. 4,684,382, the evaporated fuel absorbed by EPDM elastomer foam evaporates to be absorbed by charcoal placed in the vicinity thereof, thus causing the charcoal to decrease its working capacity.